1. Field of the Invention
The present invention relates to the bonding of metallic leads to pads of integrated circuit chips, and more particularly to an improved method and apparatus that applies force at different rates over predetermined periods of time to minimize chip fracture.
2. Description of the Prior Art
Heretofore, lead frames were bonded to metallic pads of integrated circuit chips by bringing a pre-heated thermode into contact with the leads, and rapidly increasing the contact force of the thermode until the bonding pressure is attained. This prior art method may be carried out by apparatus 10 schematically illustrated in FIG. 1 which includes a bonding thermode 11 supported by a small cylinder 12 that is fitted with a diaphragm 13. Above the diaphragm 13 is a chamber 14 that is under air pressure P2 supplied by line 15 through a pressure regulator 16. The cylinder 12 is supported by a larger cylinder 17 by way of a diaphragm mounted piston rod 18, above which is an air chamber 19. The air pressure for the chamber 19 is supplied at the inlet 15 to a pressure regulator 20 that is set at a pressure P1. The air is admitted to the chamber 19 through a solenoid valve 21 and an adjustable orifice 22. The regulator 20 and the adjustable orifice 22 provide some degree of control over the speed at which the small cylinder 12 is pushed downwardly to place the thermode 11 into force contact with the lead frame 23, which is in alignment with an integrated circuit chip 24.
To carry out the bonding sequence of the prior art, an operator activates the solenoid valve 21 causing air pressure from the inlet 15, the regulator 20 and the orifice 22 to force the thermode 11 to contact the lead frame 23. When this contact occurs, the motion of the diaphragm 13 and thermode 11 stops. However, the cylinder 12 moves slightly relative to the thermode 11 to operate a switch 25 that starts a timer 26. At the expiration of a predetermined length of time, sufficient to effect the bonding, the timer 26 operates the solenoid valve 21 to release the air pressure from the chamber 19 by way of vent 27. As a consequence, the small cylinder 12 carrying the thermode 11 is pulled upwardly away from the chip 24 by the springs 28 in preparation for the bonding of another chip.
As shown in the prior art drawing of FIG. 1, the leads of the frame 23 may have raised bumps 30 which align with the bonding pads 31 of the chip 24. The leads 23 and bumps 30 may be typically made of gold plated copper and the bonding pads 31 may be aluminum. The temperature of the thermode 11 is controlled to be high enough to anneal the bumps so they can deform slightly, and evenly distribute the bonding pressure over the interface between the bumps 30 and pads 31, in an attempt to prevent fractures in the chip 24 under the pads 31.
The contacting of the lead frame 23 by the thermode 11 causes the temperature of the lead frame 23 and chip 24 to increase as a function of time. The operation of the apparatus 10 causes the pressure of the interface between the leads 23 and bonding pads 31 also to increase as a function of time. As a result, the compliant mass of metal anneals and the gold plate of the bump 30 forms an alloy with the aluminum pad 31 to create a bond between the two.
The mechanical force applied to the lead frame 23 is preferably sufficient to compress or deform the bumps approximately 2.times.10.sup.-4 of an inch. Each of the bumps 30 should receive the same degree of deformation with a tolerance of not more than 1.times.10.sup.-4 of an inch. However, to accomplish such a precise bonding force, it is required also that the surface supporting the chip and the abutting surface of the thermode be parallel.
The method and apparatus of the prior art previously described is adequate to effect satisfactory bonds for the most part, but during the bonding process, the pressure at the interface (leads 30 and pads 31) tends to increase to the extent that full bonding pressure is reached before annealing temperature, resulting in a high incidence of chip fracture. Also, a slight misalignment of the apparatus causes insufficient bonding pressure at one side of the frame 23, and too much pressure at the other side, resulting in non-uniform deformation of the bumps. It would appear obvious that the problem of chip fracture could be overcome merely by increasing the temperature of the thermode 11, so that the chip reaches annealing temperature prior to or at the time the thermode reaches full bonding pressure. However, as is the case with too great a pressure before optimum temperature is reached, too high a temperature also results in chip fracture. Another solution involved preheating the chip on a hot plate prior to bonding, but this accelerates the formation of aluminum oxide before bonding which is undesirable.
Therefore, it is desirable to provide an improved method and apparatus for gang bonding lead frames to integrated circuit chip pads that results in improved joints, and does not cause a high incidence of chip fracture.